Optical fiber cables of the type to which the invention relates are well known in the art. See, for example, U.S. Pat. Nos. 5,229,851; 5,531,064 and 5,621,841.
In such cables, there is a central strength or structural member at the axis of the cable around which a plurality of buffer tubes of plastic loosely receiving optical fiber ribbons are wound. The buffer tubes are encircled by one or more layers of plastic and/or metal.
In the optical fiber cable structure heretofore described, the general practice in the prior art was to make buffer tubes from polybutylene terephthalate (PBT), polycarbonate (PC), a layered combination of PBT and PC, or a polyamide such as Nylon-12. These materials are good materials for making buffer tubes because they have high Young's modulus and low thermal expansion coefficients. However, such materials are costly and have low flexibility and moisture sensitivity, and cause difficulty in handling and processing due to the mechanical properties of the materials. For example, PBT has a glass transition temperature of +50.degree. C. and can kink relatively easily during handling, e.g. when bent to a radius less than three inches as may be done in a typical splice enclosure. Thus, it is often necessary to remove the buffer tubes in order to coil the optical fibers. Polypropylene without a copolymer has a glass transition temperature of -15.degree. C.
More recently, polypropylene-polyethylene (PPC) copolymers have been used as buffer tubes to remedy the listed deficiencies of the prior art materials. See, for example, U.S. Pat. No. 5,574,816 which is incorporated herein by reference. High density polyethylene (HDPE) has also been employed as a buffer tube material in the prior art. One especially attractive feature of both HDPE and polypropylene-polyethylene copolymers in buffer tubes is that they are flexible even at low temperature, i.e. below -40.degree. C. HDPE has a glass transition temperature of -76.degree. C., and while polypropylene itself has a glass transition temperature of -15.degree. C., the glass transition temperature of PPC can be lower. However, buffer tubes made from such materials are typically not as strong as the prior art tubes, such as tubes of PBT, and require thicker walls in order to resist crushing pressures or it would be expected that the clearance between the optical fiber ribbon stack and the interior surface of the buffer tube should be increased. Thus, in either event, a larger cable diameter would appear to be necessary for fiber counts comparable to cables with buffer tubes of less flexible materials.
Large fiber count cables are often installed in buried ducts, and therefore, cable diameter is of substantial concern. Cable companies which install optical fiber cables desire the maximum fiber count in the minimum duct size without sacrificing other properties such as flexibility and ease of midspan access.
While it is possible to provide high fiber count optical fiber cables which will have the required diameter when the buffer tube wall can be relatively thin, e.g. the buffer tube is made of a plastic such as PBT which has the undesirable properties set forth hereinbefore, the use of other materials, such as HDPE and polypropylene-polyethylene copolymers (PPC), which have desirable properties, has caused problems in providing an optical fiber count comparable with the fiber count of the prior art for the same cable diameter.
For example, it has been possible with the prior art buffer tube materials to provide an optical fiber cable with 864 optical fibers (6 buffer tubes with a 12 ribbon stack, each ribbon having 12 optical fibers) which can be used in a duct of 1.25 inch diameter. Also, it has been possible to provide an optical fiber cable with 432 (6 buffer tubes with a six ribbon stack, each ribbon having 12 optical fibers) which can be used in a one inch duct. It is desirable that an optical fiber cable with buffer tubes formed from the more flexible plastics have the same fiber counts and fit into such ducts.
It is known in the art that the transmission properties of optical fibers are affected by many factors including the lay length of the buffer tubes, the pitch at which the optical fiber ribbons in the buffer tubes are wound and the ratio of the buffer tube bore diameter to the cross-section of the ribbon stack. See, for example, the above-identified U.S. Pat. Nos. 5,531,064 and 5,621,841 and prior art cited therein.
U.S. Pat. No. 5,531,064 states that it reduces transmission losses when the buffer tube plastic is PBT by controlling the clearance between the ribbon stack and S-Z twisting both the buffer tubes and the ribbon units in a certain relation. For the reason set forth hereinbefore, a buffer tube made by PBT can have a relatively thin wall and for a given cable diameter, a wider choice of clearance can be available than is the case when the buffer tube is made of HDPE or PPC. However, the cable is less flexible, particularly at low temperatures and has other undesirable properties. In addition, manufacturing cost and difficulty is increased by having to S-Z strand the buffer tubes and to S-Z twist the ribbon stack in a predetermined relation.
Another concern for buried optical fiber cables and such cables routed in crowded ducts is the need for such cables to be locatable in order to repair damage or to access optical fibers for routing purposes. Trying to distinguish between similar cables in a duct, or to find the exact cable location in buried earth can be a haphazard task if there is no means for remote identification of the cables. If the incorrect cable is opened, thousand of communications lines may be interrupted, causing great inconvenience to users and substantial financial damage to communications providers.
Communications providers have solved the cable location problem by buying cables with a metallic armor incorporated in the cable under an outer jacket. The metallic armor was used as a conductor for an electrical signal for cable locating purposes. This solution is practical only if the cable application requires armor for moisture or mechanical protection. In applications where armor is not normally employed, the armor makes the cable heavier, bulkier, less flexible, and more expensive.
Other communications providers have run tracer wire inside the ducts used to carry the cables. This practice, however, adds labor to cable installation.
More recently, cable manufacturers have included magnetic particles in the cable construction, such magnetic particles being detectable above ground for a buried cable. See, for example, U.S. Pat. Nos. 5,577,147; 5,305,410; 5,305,411 and 5,636,305. However, the magnetic particles do not have the flexibility that an electrical signal has for distinguishing one cable from another. Further, the magnetic particles are not an inexpensive solution with respect to material costs.
What is needed and is apparently lacking in the art is a small diameter, high fiber count, optical fiber cable with low optical signal attenuation which employs buffer tubes of greater flexibility, particularly at low temperature, and which can also include desirable features known in the art such as "dry" water blocking provisions. It would also be advantageous if such cables in buried ducts were locatable from above ground without substantially increasing cable cost or manufacturing processes.